Uracil DNA glycosylase (UNG) is an enzyme that removes misincorporated and mutagenic uracils, leaving an abasic site typically repaired by the host base excision repair pathway. Each herpesvirus encodes a viral UNG, but we know little regarding the function of this highly conserved gene in the virus lifecycle. Studies indicate that herpesvirus vUNGs interact with components of the viral DNA replication machinery and promote late stage replication events in primary or quiescent cells. The vUNG of herpes simplex virus has been reported to enhance herpes simplex virus latency in mice, but the role of vUNG in a full pathogenic course of gammaherpesvirus infection in a natural host has not been reported. The gammaherpesviruses Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, and the mouse model pathogen murine gammaherpesvirus 68 establish latency in immunoglobulin class-switched B cells; infection can lead to immortalization and transformation of B cells. Host UNG2 activity is required for generating the diverse antibody repertoire of B cells, but also triggers chromosome translocations that are the etiology of lymphomas. Thus, alterations of uracil glycoslyase activity may tip the balance between immunity and malignancy. This proposal is to understand how the UNGs encoded by the gammaherpesviruses promote viral pathogenesis and impact the B cell antibody repertoire and lymphomagenesis of the host. In Aim 1, the impact of viral and host UNG on promoting the integrity of the viral genome in the process of lytic replication will be determined. In addition, we will examine changes in the pathogenesis of MHV68vUNG in the presence or absence of host UNG2 to determine the contribution of viral and host UNG to replication, latency, and reactivation in vivo. In Aim 2, we will address whether viral UNG promotes mutagenic outcomes such as immunoglobulin gene class-switch recombination and somatic hypermutation, off-target mutations in oncogenes, or translocations such as c-myc/Ig translocations that define EBV+ Burkitt's lymphoma. Our studies might uncover an interaction of viral UNG with the host cytidine deaminase AID that will provide critical insight regarding the molecular etiology of HV cancers.